Rockstar games announced that last GTAIV trailer will come out thursday 27 march 2008. Rockstar said that after this trailer, only thing what is coming out is the full game himself.

BTW, trailer name is "Good Lord, What Are You Doing" aka "Everyone's A Rat"

EDIT: Trailer is out and available to look at http://www.gamespot.com/video/0/6188467/videoplayerpop?